The present invention relates to an electromagnetic control valve.
A general or usual clutch device for tractors comprises a forward clutch mechanism and a rearward clutch mechanism hydraulically controlled respectively by a pair of electromagnetic proportional pressure control valves which are independent of each other. By the way, should the forward and rearward clutch mechanisms be brought simultaneously to their respective ON positions, such an inconvenience would arise that a gear train associated with the clutch mechanisms is destroyed. Accordingly, the arrangement is such that a control circuit having a microcomputer or the like is provided for prohibiting that both the clutch mechanisms are brought simultaneously to their respective ON positions. With such arrangement, however, there may be a case where the fine or minute dust enters an interface between a spool of, for example, one control valve and a housing thereof within which the spool is accommodated for axial movement, so that the spool is made immovable and, as a result, the clutch mechanism associated with the one control valve can no longer be switched from the ON position to the OFF position, in spite of the fact that the control circuit brings the one control valve to the unexcited state. In this case, when the clutch mechanism associated with the one control valve is in its ON position, regular or normal operation of the electromagnetic valve associated with the other clutch mechanism brings the other clutch mechanism to its ON position, thereby causing the above-mentioned inconvenience.
As the prior art of the present invention, an electromagnetic control valve is known, in which a plurality of spools are accommodated within a single housing. The spools are controlled in movement respectively by a plurality of electromagnetic drive means to control respective flow passageway means formed in the housing correspondingly respectively to the spools, in turn, to control a plurality of subjects to be controlled which are associated respectively with the flow passageway means. Since the spools in this control valve are not arranged in coaxial relation to each other and do not interfere mechanically with each other, the control valve is different from a control valve according to the present invention subsequently to be described.
Japanese Patent Application Laid-Open No. 59-58285 discloses, in FIG. 2, an electromagnetic proportional pressure control valve in which a pair of spools are accommodated within a housing in coaxial relation to each other. These spools are controlled respectively by a pair of electromagnetic drive means which are independent of each other. A pair of stoppers are arranged between inner ends of the respective spools which face toward each other. The inner end of each of the pair of spools is engageable with a corresponding one of the pair of stoppers, so that movement of the spool toward the center of the housing is restricted by the stopper. The control valve disclosed in the above Japanese patent is different from that according to the present invention in that the pair of spools have their respective moving regions which are spaced away from each other, are independent of each other and do not interfere mechanically with each other, and that the pair of spools control a single subject.
Japanese Patent Application Laid-Open No. 55-14360 which corresponds to U.S. Pat. Nos. 4,281,682 and 4,282,901, and Japanese Patent Application Laid-Open No. 62-98409 disclose an electromagnetic proportional pressure control valve in which a pair of spools are accommodated within a housing in coaxial relation to each other. In this control valve, the pair of spools have their respective inner ends which face toward each other in spaced relation or through a coil spring. The control valve disclosed in the above patents is different from that according to the present invention in that moving regions of the respective spools are not overlapped with each other and do not interfere mechanically with each other, and that the pair of spools are controlled by a single electromagnetic drive means, as a consequence, to control a single subject.